Membrane bioreactors are designed to remove contaminants from wastewater. This wastewater can come from residences, businesses, industries, and runoff water from rainfall and/or flooding. A membrane bioreactor (“MBR”) employs micro-filters (membranes) that separate clean water from solid waste, sludge, and other contaminants. First, the water is biologically treated in an aerobic process by micro-organisms that consume the organic contaminants in the water and produce solid waste sludge. Then, the water is drawn through the micro-filters, which separate the water from the sludge. The clean water is then pumped out of the micro-filters.
Current commercial membranes used in membrane bioreactors are composed of a polyvinylidene fluoride (PVDF) barrier layer and a PET non-woven fabric substrate, both of which materials are non-elastic or have little elasticity. Layers of fouling particles will often build on the membrane surface, so keeping these membranes clean is a constant concern. Fouling can be divided into reversible and irreversible fouling, based on the attachment strength of particles to the membrane surface. Membrane fouling that can be removed by hydraulic cleaning, such as backwashing and/or gas scouring, is defined as reversible fouling. A backwashing process involves reversing filtrate flow to dislodge foulants that have accumulated on the membrane surface and in the pores.
For currently available membranes, only the loosely attached foulants (i.e., sludge flocs and colloids with larger sizes than the pore sizes of the membrane), can be removed during the backwash process. By contrast, those particles, colloids and solutes that are comparable or smaller than the pore sizes can easily get stuck in the pores or be attached to the pore wall, and cannot be removed by simple physical cleaning. In this case, harsh chemical treatment is often used to cleanse the fouling. However, repeated uses of aggressive chemical cleaning can accelerate membrane deterioration and shorten the membrane lifetime. In addition, the disposal of toxic and caustic spent chemicals is an environmental problem itself. More importantly, it has been demonstrated that even for long term chemical cleaning, only the fouling on the surface of the membrane can be removed, while the fouling that is caused by depth filtration still remains in the membrane, making the fouling irreversible.
Improved membranes for use in MBR systems remain desirable.